PHOTOS: NASA's Darkest Moments

NASA marks anniversaries of Challenger, Columbia, Apollo 1 accidents.

PHOTOS: Challenger, Columbia, Apollo: NASA's Darkest Moments

The end of January and start of February are always a tough time for NASA. Jan. 27 is the anniversary of the Apollo 1 fire in 1967, Feb. 1 marks the loss of the space shuttle Columbia in 2003, and Jan. 28, 1986 is the day Challenger (above) exploded. They are the only three cases in which American astronauts have died in their spacecraft. (NASA)